1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. Note that in this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, for example, a transistor (a thin film transistor and the like) can be given. In addition, a semiconductor device also refers to a display device such as a liquid crystal display device.
2. Description of the Related Art
In recent years, thin film transistors (TFTs) over a substrate having an insulating surface (e.g., a glass substrate) have been attracting attention. TFTs are formed using a semiconductor thin film with a thickness of several nanometers to several hundreds of nanometers. As a semiconductor thin film for forming the TFTs, an amorphous semiconductor film or a polycrystalline semiconductor film is mainly used. Further, a microcrystalline semiconductor film is also known (e.g., Patent Document 1).
A threshold voltage is one of important parameters which determine performance of TFTs. For example, in the case of an n-channel TFT, when a gate voltage is zero, a TFT having a threshold voltage higher than 0 V (which is normally-off) does not turn on and almost no current flows between a source and a drain. On the other hand, when a gate voltage is zero, a TFT having a threshold voltage lower than 0 V (which is normally-on) turns on and current flows between a source and a drain. Therefore, it is preferable that threshold voltage of a TFT as a switching element is high. A number of attempts has been made so far to adjust the threshold voltage of TFTs (e.g., Patent Document 2).